Formation of Internal Transport Barriers and lts lmpact on the JT-60U Plasmas

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1 J. Plasma Fusion Res. SRS, Vol.4 (21) Formation of nternal Transport Barriers and lts lmpact on the JT-6U Plasmas D Shunsuke+ and the JT-6 Team Naka Fusion Research stablishment, Japan Atomic nergy Research nstitute, baraki, j -193, Japan (Received: 5 December 2 / Accepted:27 October 21) Abstract Formation of internal transport barriers (TBs) in a plasma is a key to access to improved confinement. n JT-6U, there are two types of plasmas which are accompanied with TB. One is a high Bo plasma and the other one is a reversed magnetic shear (RS) plasma. n RS plasmas, interesting features can be seen in spatial profiles of plasma parameters. For example, the location of the TB can be strongly related to the location where the safety factor (4) becomes minimum, and the radial electric field (8,) has strong spatial variation at the TB. These features can be related to the physics mechanisms of confinement improvement at the TB. On JT-6U RS plasmas, experiments to modify the toroidal current density or the toroidal rotation profiles, which are expected to influence the TB structure directly or indirectly, have been carried out. And these modifications were fond to cause chanses in the TB structure. The results are shown in this paper. Keywords: JT-6U, TB, reversed magnetic shear, LHCD, toroidal rotation 1. ntroduction The main objectives of the JT-6U project are studies on confinement, stability, steady state operation, heat and particle handling by a divertor configuration and understanding of underlaying physics in order to establish physics basis for the international thermonuclear experimental reactor (TR) and steady state reactor such as SSTR [1]. For this purpose high performance plasmas, such as H-mode, high Bo t2j, reversed magnetic shear (RS) plasmas [3] and the latter two combined with the H-mode [4,5], have been developed. What is essential to keep improved confinement in those plasmas is to maintain the internal and/or the edge transport barriers (TB t3,61 and/or TB). specially a role played by the TBs is a key to access high confinement. The formation of the TB and its dynamics can be seen more clearly in the RS plasmas than those * C o r r e s p ondin g autho r' s e - mail : naka.j ae ri. g o.j p in the high o plasmas in JT-6U. Therefore, topics concerning to the TB structure and its modification in the JT-6U RS plasmas are reported in this paper. n the second section, the tokamak, the heating system and diagnostics which are closely related to RS experiments shown in this paper are briefly described. n the third section, some general characteristics of the TBs in the JT-6U RS plasmas are introduced. Also some typical achievements in performance so far in the JT-6U RS plasmas are shown. Following that, in the fourth section, experimental results on toroidal current density (j) profile modification via lower hybrid current drive (LHCD) and its impact to confinement are presented. n the fifth section, experimental results on relation between the TB gradient and the radial electric field profile modification via external torque input control by The Japan Society of Plasma Science and Nuclear Fusion Research 99

2 D S. et al., Formation of nternal Transport Barriers and its mpact on the JT-6U Plasmas shown. Lastly, the summary will be presented in the last section. 2. The Tokamak and the Heating/Current Drive and Diagnostics Systems The JT-6U tokamak is a single null divertor tokamak device, the plasma major radius (Ro) m, the plasma minor radius (ar) m and the maximum toroidal magnetic field (,B,) <- 4 T at R = 3.32 m. On the tokamak a variety of heating and current drive systems are equipped. They are the neutral beam injection (NB), the lower hybrid range of frequencies (LHRF), the electron cyclotron range of frequencies (CRF) and the ion cyclotron range of frequencies (CRF) systems. Two out of the four systems, the NB and the LHRF systems, were mainly used in the experiments described in this paper. Detail of the two systems are: l) The NB system: Two types of ion sources are used, one is based on standard positive ion based ion source of which acceleration voltage is 3 1 kev and the other one is based on negative ion based ion source of which acceleration voltage is < 5 kev. The purposes of NBs are heating, toroidal torque input and current drive. Due to higher acceleration voltage, the negative ion source based NB (N-NB) is better in driving current. 2) The LHRF systems: Two types of multi-junction (MJ) LHRF launchers are installed. One which is installed in a port at poloidal angle = 45o, consists of four rows of eight MJ modules which have 3 subwaveguides. The other one, installed in a horizontal port, consists of two rows of four MJ modules which have 2 sub-waveguides. Key diagnostics indispensable to this paper are, the mortional Stark effect (MS) measurement for the safety factor (q) or j profile measurement, Thomson scattering measurement using Ruby and YAG lasers for the electron temperature (7") and the density (n.) profile measurement, and charge exchange recombination spectroscopy (CXRS) measurement for the ion temperature (4) and the toroidal rotation (Vo) profiles measurement. 3. General Characteristics of Reversed Magnetic Shear Plasmas in JT-6U n an RS plasma, thet profile is a hollow one. Then the 4 profile comes to have a minimum inside a plasma. n this paper, the minimum value in the q profile and its location are denoted os 4-in and po-1". With this q profile, the magnetic shear s = plq dqldp becomes negative inside pq-in. Here p is the normalized flux radius. n JT-6U experiments, an RS configuration is usually formed by NB heating at fast plasma current (1) ramp-up to retard current penetration by raising 7" and with help of the bootstrap current. However it should be noted here that LHRF injection at the p ramp phase also could form an RS configuration with quite a larger po-6 [7]. Moreover, even at the current flat-top an RS configuration formation was observed with LHCD or CCD although pq.rn is not very large. Usually a moderate to a high performance RS plasma is formed by injecting adequate NB power into an RS configuration. Typical profiles in an RS plasma are shown in Fig. 1. As shown in the top box, the 4 q T1 (kev) u 1.5 p TB foot ne 2 ('l1s1n-e o1t evim; -4 Fig. 1 Plasma parameter profiles in a typical RS plasma in JT-6U. The top box; the q profile and the minimum (9.,") in the profile and its location (po.in) are schematically shown. The second box; the 4, and n. profiles. The bottom box; the 7" and the /i profiles with the neoclassical thermal diffusivity profile for a reference, and the, profile. The region of the TB is schematically shown with the gray area. Also the TB "foot" location is also indicated schematically by an arrow. 1

3 (RrD s. et al., Formation of nternal rransport Baniers and its mpact on the JT-6U plasmas profile is increasing toward the center from pn.;. n the second box the 7", the Ti and the n. profiles are plotted. At a certain position in a plasma the gradient of all these profiles changes quickly, and the profiles build up toward the center. The position is referred to as the TB "foot" (pioo,) in this paper. t should be noted that although ppool could vary in each profile somehow it is found to be strongly related to pn^6 in the JT-6U RS plasmas [8]. However this steep gradient in the profiles usually does not continue up to the center, but becomes much less steep, or even flat, at a certain point (referred to as the TB shoulder). A region between the TB foot and the shoulder is called the TB region. As shown in the bottom box, both the electron and the ion thermal diffusivities (p and 1) show sudden drop at this region. Actually in this case, 7, is found to be as small as the neo-classical value as shown in the figure. However, as expected from the less steep temperature and density profiles, the thermal diffusivities go up inside the TB shoulder. Also, the radial electric field (,) profile is shown in the bottom box and indicates strong spatial variation at the TB region. Formation of the TB can bring benefits from a view point of steady state advanced tokamak operation, such as high confinement and large bootstrap current fraction to 1o ("fus). n JT-6U, there have been many achievement in view of plasma performance owing to TB formation. Some of them are brieflv described as follow; l) High equivalent D-T fusion gain (81) The RS plasmas in JT-6U have been proved to have potential to achieve high fusion plasma performance, and in the campaign in 1998 the value of Q# has reached up to 1.25 at p = 2.6 MA and the toroidal magnetic field at the plasma center (8,q) = 4.4 T l5l. The edge of the RS plasma was in the L-mode. n order to obtain high performance, it was found to be advantageous to form the TB early in the 1o ramp-up phase. n order to maintain the TB at large radial location which gives better performance, a certain amount of power needs to be applied. Otherwise, the TB shrinks and the performance becomes lower or is even lost. On the other hand, since the current keeps penetrating into the plasma center, the minimum of the safety factor (4) profile (4-1) keeps decreasing. 2) High confinement RS plasma with H-mode edge One of the key concept in the advanced tokamak operation is to utilize bootstrap current as much as possible for non-inductive operation [1]. On JT-6U, optimization of an RS plasma has been pursued to raise cn o!, lrj C' = s -.4. Bp-o n T r r o.4.8 R"t;"/a Fig. 2 The MS polarization angle ly*"t - BrlBry vs radial location (distance from the magnetic axis normalized by the minor radius) of the measurement, Almost zero yms in the central region suggests almost zero Bo or j. /s5. This effort has succeeded in producing an LMy H- mode RS plasma which has a large/", of about 82o, with 1o =.8 MA, B,o =3.4 T, q at 95Vo flux (Qs) = 9.3 and the triangularity (d) =.43 [9]. n the plasma, the confinement enhancement factor to the H-mode thermal confinement scaling (HHgarv,zt) reached , and that to the L-mode scaling (F1sqp) was , with the normalized p (8") = Owning to the large/r. of 8Vo and the P-NB driving the rest 2Vo of 1, full current drive was maintained with such a highly improved confinement state for 2.7 s (about 6 x r"). n addition to the performance improvement shown above, a very interesting structure is found in the RS plasmas, that is a current hole [1,11]. n an RS plasma, almost zero poloidal field (Bo) is observed in the central region (Fig. 2). This in other words means that j is very small or even zero in the region. t should be stressed that this finding suggests a possibility of a stable tokamak operation with very small or even zero toroidal current in the core region. 4. Modification of the Current Profile and the TB Location As noted in the previous section, the TB foot is found to be strongly related to the position of 4-,n in JT- 6U RS plasmas. t was also found that expansion of Pq-in (or - Proo,) gives improvement of confinement performance [12]. Therefore it is an interesting question what happens when the q-1, location is changed via some active external scheme. Modification of the TB lt

4 D S. et al., Formation of nternal Transport Barriers and its mpact on the JT-6U Plasmas location by modifying target q profile by means of LHCD was already reported [7]. Also quasi-steady state sustainment of the RS configuration and TB by external LHCD was demonstrated [13]. On the other hand, quasisteady sustainment of the RS configuration and TB with high bootstrap current fraction (- SOVo) was also reported as described in the previous section [12]. However, in the former cases the bootstrap current fraction was not as high as expected in the future machines, and in the latter case there was small room for external NBCD to work and also 4e5 is too high in view of economical operation of a future rector. Therefore, in order to explore current drive and cultent modification effect with high (but not too high) bootstrap current fraction at lower 4e5, LHCD was applied to high BN RS plasmas at low qs5rcgime. The experiment was done at Bto = 2.5 T with deuterium gas. The waveforms of typical plasma parameters are shown in Fig. 3. As shown in the figure, the target RS plasma was formed by P-NB heating. Then Pys was reduced and kept constant for clearer measurement of the LHCD effect. t is noted that just at the PNs step-down a mini collapse occurred and a large drop in Au at s is attributed to the mini collapse. After P111 injection, the surface loop voltage (yt ) started decreasing, which indicated that the inductively driven plasma current was being replaced with non-inductive current, that is LH driven current. n the bottom box n" at p - O.2,.6 and.8 are shown, which correspond to the central region, near the TB foot and peripheral region respectively. The temporal evolution of n"at p -.6 starts increasing at about 6.7 s, which indicates this position is coming into the TB region, or in other words the TB region is expanding. Change in the profiles is shown in Fig. 4. n Fig. 4 (a), the 4 profiles at 6.4 and 7.5 s are shown. As shown in the figure, the location of the 4.6 shifts outward. This shift is attributed to off-axis LHCD. n Fig. 4 (b),n. profiles at 6.5 and 7.5 s are plotted. As mentioned before, expansion of the TB foot is seen in the change in the profile. t is noted that although the central density decreases, the expansion of the TB foot compensates to keep high electron population in the core region. Furthermore it should be noted that expansion of the TBs in the Z" and T1 profiles is also observed. n the discharge, V t rcached very closed to zero but not completely, as shown in Fig. 3. Demonstration of full current drive in this regime is very interesting and important in relevance to the TR steady state operation. n order to access full current drive, 1o was,1 (mhlo's fu1 v, o'5 (v) o 6 ne4 (1tsm-o;2 Fig. 3 A typical waveforms of LHCD on a high p" RS plasma at /D = 1 MA and B,= 2'5 T. The LH power was modulated to 1% with the dutv of 9%. 6 cr4 (D = L CD of g o Time {s) 6 p.6 f a o \ 2A on Pua ;-(Mw) 5 Pr-n (Mw).5 1 p Fig. 4 Profiles of q and n" at 6,4 s/6.5 s and 7'5 s (solid curve, circles). reduced to.9 MA and also N-NB was injected. The role of N-NB injection is not only current drive but also effective central heating. The waveform ofthe discharge is shown in Fig. 5. njection scenario of P-NB and LHW is similar to that shown previously, but N-NB injection started prior to LHCD by.2 s. As shown in the fitgure' V7 dropped to zero and then even negative, showing full current or even over drive. Similar to the previous case, temporal evolution of n" near the TB foot suggests expansion of the TB foot. As the results of the \ \./ '1 A J t/l fl q a 7 q n b'! \ J :r? 1 K '1 to2

5 lp (MA} Prt (Mw) vl (v) o'u tf F = 4l 2l H.5 f. L r- 4 ne 119m-3) e'f. tj 5.5 DS. et al., Formation of nternal rransport Barriers and its mpact on the JT-6u plasmas Time(s) 1 PilB o (Mvv) 3 2 Woi" 1 {MJ) (v) z C\ b Momentum Source Fig. 5 Wave forms of the discharge in which full current drive was achieved simultaneously with high confinement (HHsae,rtl at high normalized electron density (n"lno*1. Also in this shot Pr" was modulated to 1% with the duty of 9%. Fig. 6 Typical torque (momentum) input profile, balanced, co- and counter directional, in the discharge shown in Figs. 7 and 8. p TB expansion, the confinement in the plasma was improved further and some of the plasma parameters reached to the region which is relevant to the TR with full current drive condition. The confinement improvement factor to the H-mode (HHs8e2) is about 1.4 and, the fraction of line averaged density (4) to the Greenwald density (z6yy) is about.82. t should be stressed that simultaneous achievement of full current drive at high confinement and high density is a new regime in JT-6U operation. Although ess = 6.9 in the discharge is still not very low, it is approaching to the range that is expected in the TR steady state operation scenario, that is 4e5 = Modification of the Toroidal Rotation Profile and the TB Strength As described in the second section, a large spatial variation in the radial electric field profile is often observed in RS plasmas in JT-6U. Theoretical predictions show that competition between the shearing rate (ars,s) and linear growth rate (y) of a microturbulence is a key in the turbulence suppression. Here, o)s,s is written [4]; d)o,o=(rb,)'+? B, dyl RBo (1) Here, B6is the poloidal magnetic field and y= constant is a magnetic flux surface. Therefore, spatial derivative of, (it will be expressed as i later on in this paper) would be a key parameter. On the other hand the radial electric field is written as; u,=)ft,+va Bo -v, Bo. Q) Here, pi, Zi, ni, Vq and Ve are the pressure, the charge number, the density, the toroidal and the poloidal rotation velocities of an ion species, and Bp is the toroidal magnetic field. As noticed in the equation, there would be a room to modify the first and the second terms externally. That is, control of the heating to modify the first term and the control of the toroidal torque input to modify the second term. On the other hand, the third term is expected to be determined mostly by neo-classical force balance. The idea is to utilize four tangential P-NB lines on JT-6U to control torque input profile to act on the radial electric field via modification in the second term in the eq. (2). The experiment was carried out with p = 1.5 MA and 8, = 3.5 T and the working gas was deuterium [8]. n the discharge, the input toroidal torque profile was changed by changing combination of tangential beam units. Three input momentum profiles that were used in this experiment are shown in Fig. 6. The combination which gives co-directional torque is referred to the "CO" injection. On the other hand, the combination which gives counter-directional torque is referred to the "CTR" injection. The combination which gives almost zero torque relative to the other two cases is referred to the "BAL" injection. The target RS plasma was initially formed with the BAL injection, then switched to the CO injection, then the CTR injection and finally to the BAL injection again. Since the interest is on the TB structure, the 13

6 D S. el al., Formation of nternal Transport Barriers and its mpact on the JT-6U Plasmas gradient of 4 at the TB is taken as a measure of the TB characteristics. n Fig. 7, the temporal evolution of the d gradient at the TB is shown, and the combination of the torque input is also shown schematically at the top of the figure. As indicated in the figure, atjust after 6.25 s the Z; gradient started decreasing, and atjust after 6.65 s the gradient started increasing again, or recovering. Note that the NB power was kept almost constant. The confinement improvement (l/sep) was evaluated as 2.2, 1.6 and 1.8 at 6.25,6.6 and 7. s respectively. The change in the toroidal rotation profile is shown in Fig. 8 (a). As shown in the figure, in the case of the CO injection Va builds up in the central region while the profile flattens at the TB foot just before the degradation in the gradient starts ( s). On the other hand, in the case of the CTR injection followed by the BAL injection, a flat V6 profile at the TB foot comes to have a positive gradient just before the recovery of the Ti gradient starts. The gradient of. at each timing was calculated and shown in Fig. 8 (b). Together with ',, gradients of the first and the second terms in eg. Q) are shown, labeled with *Vp" and Vq respectively. As seen in Fig. 8 (b), towards the degradation in i gradient,what changes most clearly in the j profile is a decrease in ', at the TB foot. On the other hand, when Z1 gradient is about to recover, increase of i at the TB foot is observed. Both ofthem are attributed to the change in the "Va" due to change in the Va profile. These results suggest that the, gradient at the TB foot is a key parameter to control the confinement of whole the TB resion. 6. Summary General feature of the reversed magnetic shear plasma in JT-6U are descried; o the location ofthe TB foot is strongly related to the location at which q becomes minimum, o strong spatial variation in the, profile is often found to be formed at the TB region,. the temperatures and density profiles often become flat in the central region. Two topics relating to the first and second feature are described in this paper. The first one is concerning to the current profile and the TB location. t was shown that by extending the location of the minimum 4 by means of non-inductive off-axis current drive using o L f c Cg (t, th] d A ll co CTA BAL o6' Time (s) Fig. 7 Temporal evolution of the fi gradient at the TB when torque input profile was changed as "CO" + "CTR" =+ "BAL". Fig. 8 (a); The V, profile at 5.9, 6.25, 6.45 and 6.6 s in the discharge shown in Fig.7. (b); The gradient of profile and its terms dominated by Yp and Vr, labeled with "Vp" and "Vr" respectively, at 5.9, 6.25,6.45 and 6.6 s in the same discharge. lower hybrid waves the location of the TB foot expanded outward. By combining the off-axis LHCD and the current drive by N-NB, full current drive of /o =.9 MA with high confinement of HHss1r21of about 1.4 at high density normalized to the Greenwald density rc"/ n61ry of about.82 and qes of 6.9. The other one is conceming to modification of the, profile via control of the toroidal torque input in order to affect the transport characteristics of the TB. t was demonstrated that by changing the toroidal torque input profile by choosing combination of four tangential NB lines the gradient at the TB changed. As the result of the modification in the Vr profile, the. profile and thus the gradient of, profile was changed. t was fond that prior to the degradation of the Zi gradient, the, gradient at the TB foot became small due to flattening of the Vr profile at the TB. On the other hand, prior to the steepening of the d gradient increase ro4

7 rd S. et al., Formation of nternal rransport Barriers and its mpact on the JT-6u plasmas of the, gradient at the TB foot was observed. These results suggest that the, gradient at the TB foot is a key parameter to control the confinement of whole the TB region. Acknowledgments The authors would like to express our gratitude to all the researchers who have contributed JT-6U under a wide range of collaborations for their valuable discussion, suggestion and comments. References t1l M. Kikuchi, Nucl. Fusion 3,265 (199). [2] S. shida, Y. Koide, T. Ozeki er a/., Phys. Rev. Lett.68, r53l (1992). t3l T. Fujita, S. de, H. Shirai er al.,phys. Rev. Lert. 78,2377 (1997\. t4l Y. Koide, S. shida, M. Kikuchi et al., in proceedings of the 15th nternational Conference on Plasma Physics and Controlled Nuclear Fusion Research, Seville, 1994 (nternational Atomic nergy Agency, Vienna, 1995), Vol. l,p.199. [5] T. Fujita, Y. Kamada, S. shida et al.,ntcl. Fusion 39, 1627 (1999). t6l Y. Koide, M. Kikuchi, M. Mori er a/., Phys. Rev. Lett.72,3662 (1994). [7] S. de et al., in the Sixteenth AA Fusion nergy Conference, Montrdal, 996 (AA, Vienna, 997), Vol.-3, pp t8l Y. Sakamoto, Y. Kamada, S. de er a/., Nucl. Fusion 41, 865 (21) [9] T. Fujita, S. de, Y. Kamada et al.,phys. Rev. Lett. 87,851 (21). [] T. Fujita, T. Oikawa, T. Suzuki et al., in Bulletin of the Japan Soc. of Plasma Sci. and Nucl. Fusion Res.,2, 27a86. l] T. Fujita, T. Oikawa, T. Suzuki er a/., Phys. Rev. Lett. 87, 2451 (21). L2T. Fujita, Y. Kamada, S. de er al., to appear in Nucl. Fusion. ll3l S. de, O. Naito, T. Oikawa er al., Nucl. Fusion 4, 3Y,445 (2). ll4l T.S. Harm and K. Burrell, Phys. Plasma 2, 1648 (1995). 15